<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text">
<input type="button" value="平方" onclick="f()">
<input type="button"  value="求和" onclick="f1()">
<div>结果</div>
<script>
  let i = document.querySelector("input");
  let d = document.querySelector("div");
  function f1(){
   // d.innerText = "结果:"+(1*i.value+1*i.value);
    d.innerText = "结果:"+(parseFloat(i.value)+parseFloat(i.value))
  }
  function f(){
  //判断用户输入的内容是否是数值  如果是NaN代表不是数值
    if(isNaN(i.value)){
      d.innerText="输入错误";
      return;
    }
  d.innerText ="结果:"+i.value*i.value;

  }

</script>
</body>
</html>